1. Field of the Invention
The present invention relates to an image forming apparatus in which paper placed on a paper feed tray is fed into the main body to form an image thereon, and more particularly to a structure of a paper feed portion for such an image forming apparatus.
2. Description of the Prior Art
Conventionally, in an image forming apparatus such as a dye-sublimation printer, sheets of paper having a predetermined shape and placed on a paper feed tray are fed into the main body one by one to form an image thereon. In this case, it is necessary to feed only one sheet of paper from a stack of paper placed on the paper feed tray.
As shown in FIG. 5, in a conventional image forming apparatus are therefore provided a paper feed roller for feeding paper and carrying force generating ribs arranged on the downstream side of the paper feed roller, across the paper feed path, to apply a predetermined carrying force to paper.
A paper feed portion of a conventional image forming apparatus is shown in FIGS. 5 (a), (b) and (c) which are, respectively, a plan view, a side elevational view, and a partial perspective view of the paper carrying surface.
As shown in FIG. 5, the conventional paper feed portion comprises a paper feed roller 1 with an axis perpendicular to the paper feed direction as well as horizontal to the ground and a carrying auxiliary portion 2 with carrying force generating ribs 20a and 20b provided thereon and positioned on the paper carrying downstream side of and below the paper feed roller 1. The conventional paper feed portion applies a carrying force to paper between the carrying force generating ribs 20a and 20b and the contact members 10a and 10b on the paper feed roller 1. This allows the paper to be fed in a predetermined direction (indicated by the large arrow in FIG. 5). In this case, the height of the carrying force generating ribs 20a and 20b is preset to generate a predetermined carrying force and also to prevent double feeding (sheets of paper from being carried in an overlapped manner).
One device for generating a predetermined carrying force for paper and for preventing double feeding in a paper feeding apparatus comprises a placing table with a recessed portion contoured to the shape of a paper feed roller mounted therein (refer to Japanese Patent Laid-Open Publication No. Hei 7-126029 for example).
It is however difficult to set optimum conditions for generating a predetermined carrying force and for preventing double feeding in a paper feed portion of a conventional image forming apparatus having such a structure. That is, increasing the carrying force makes double feeding more likely, while configuring the structure to prevent double feeding reduces the carrying force.
Although there exists an apparatus in which the height of ribs can be varied to obtain optimum conditions for the generation of a carrying force and for the prevention of double feeding, it has a complex structure, resulting in an increase in cost.